Optical reception apparatus and monitor signal generating method

ABSTRACT

An optical reception apparatus ( 1 ) of the present invention includes: a local oscillator ( 11 ) outputting local oscillation light ( 22 ); an optical mixer ( 12 ) receiving a multiplexed optical signal ( 21 ) and the local oscillation light, and selectively outputting an optical signal ( 23 ) corresponding to the wavelength of the local oscillation light from the multiplexed optical signal; a photoelectric converter ( 13 ) converting the optical signal ( 23 ) output from the optical mixer into an electric signal ( 24 ); a variable gain amplifier ( 15 ) amplifying the electric signal ( 24 ) to generate an output signal ( 25 ) whose output amplitude is amplified to a certain level; a gain control signal generating circuit ( 16 ) generating a gain control signal ( 26 ) for controlling the gain of the variable gain amplifier ( 15 ); and a monitor signal generating unit ( 17 ) generating a monitor signal ( 27 ) corresponding to the power of the optical signal ( 23 ) using the gain control signal ( 26 ).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. Pat.Application Ser. No. 17/689,030 filed on Mar. 8, 2022, which is acontinuation application of U.S. Pat. Application Ser. No. 17/060,364filed on Oct. 1, 2020, which issued as U.S. Pat. No. 11,290,202, whichis a continuation application of U.S. Pat. Application Ser. No.16/741,496 filed on Jan. 13, 2020, which issued as U.S. Pat. No.10,826,642, which is a continuation application of U.S. Pat. ApplicationSer. No. 16/219,216 filed on Dec. 13, 2018, which issued as U.S. Pat.No. 10,554,323, which is a continuation application of U.S. Pat.Application Ser. No. 15/601,103 filed on May 22, 2017, which issued asU.S. Pat. No. 10,187,174, which is a continuation application of U.S.Pat. Application Ser. No. 14/904,031 filed on Jan. 8, 2016, which issuedas U.S. Pat. No. 9,692,545, which is a National Stage Entry ofinternational application PCT/JP2014/001793 filed on Mar. 27, 2014,which claims the benefit of priority from Japanese Patent Application2013-145238 filed on Jul. 11, 2013, the disclosures of all of which areincorporated in their entirety by reference herein.

TECHNICAL FIELD

The present invention relates to an optical reception apparatus and amonitor signal generating method, and particularly to an opticalreception apparatus and a monitor signal generating method using thecoherent light transmission scheme.

BACKGROUND ART

The wavelength division multiplexing (WDM) communication belongs to theoptical communication technology. In the wavelength divisionmultiplexing communication, since a multiplexed optical signal in whichoptical signals of a plurality of wavelengths are multiplexed is used,large-volume information can be transmitted with a single optical fiber.Further, there is a technique of selectively extracting a particularoptical signal from the multiplexed optical signal, which is referred toas the coherent light transmission scheme. In the coherent lighttransmission scheme, by allowing the multiplexed optical signal andlocal oscillation light to interfere with each other and performing acoherent detection, an optical signal corresponding to the wavelength ofthe local oscillation light is selectively extracted from themultiplexed optical signal.

Patent Literatures 1 and 2 each disclose a technique relating to thecoherent light transmission scheme. Patent Literature 1 discloses atechnique of stabilizing the absolute wavelength of a local oscillationlight source and a transmission light source, thereby making it easierto set the wavelength. Patent Literature 2 discloses a technique forimproving the S/N ratio in the reception characteristic whilesuppressing an increase in costs, even in the case where a multiplexedoptical signal is selectively received by the wavelength of localoscillation light.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Unexamined Patent Application    Publication No. H04-212530-   Patent Literature 2: Japanese Unexamined Patent Application    Publication No. 2012-070234

SUMMARY OF INVENTION Technical Problem

In the coherent light transmission scheme disclosed in PatentLiteratures 1 and 2, a multiplexed optical signal transmitted from anoptical transmission apparatus is received using an optical receptionapparatus. For example, the power of the multiplexed optical signalinput to the optical reception apparatus can be monitored by branchingthe multiplexed optical signal input to the optical reception apparatuswith an optical coupler or the like, and converting the branchedmultiplexed optical signal to an electric signal with a monitor-purposephotoelectric converter.

However, since a multiplexed optical signal is an optical signal inwhich optical signals of a plurality of wavelengths are multiplexed,when a multiplexed optical signal input to the optical receptionapparatus is monitored, all the optical signals input to the opticalreception apparatus are monitored. Hence, in this case, there is aproblem that the power of an optical signal of a particular wavelengthcannot be measured solely.

In view of the problem described above, an object of the presentinvention is to provide an optical reception apparatus and a monitorsignal generating method, with which the power of an optical signal of aparticular wavelength can be monitored.

Solution to Problem

An optical reception apparatus of the present invention includes:

-   a local oscillator outputting local oscillation light having a    prescribed wavelength;-   an optical mixer receiving a multiplexed optical signal in which    optical signals being different in wavelength from each other are    multiplexed and the local oscillation light, and selectively    outputting an optical signal corresponding to the wavelength of the    local oscillation light from the multiplexed optical signal;-   a photoelectric converter converting the optical signal output from    the optical mixer to an electric signal;-   a variable gain amplifier amplifying the electric signal converted    by the photoelectric converter, to generate an output signal whose    output amplitude is amplified to a certain level;-   a gain control signal generating circuit generating a gain control    signal for controlling a gain of the variable gain amplifier; and-   a monitor signal generating unit generating, using the gain control    signal, a monitor signal corresponding to power of the optical    signal output from the optical mixer.

A monitor signal generating method of the present invention is a monitorsignal generating method for generating a monitor signal correspondingto power of an optical signal received by an optical receptionapparatus, the method including:

-   causing a multiplexed optical signal in which optical signals being    different in wavelength from each other are multiplexed and local    oscillation light having a prescribed wavelength to interfere with    each other, to extract an optical signal corresponding to the    wavelength of the local oscillation light from the multiplexed    optical signal;-   converting the extracted optical signal into an electric signal;-   amplifying the electric signal using a variable gain amplifier, to    generate an output signal whose output amplitude is amplified to a    certain level;-   generating a gain control signal for controlling a gain of the    variable gain amplifier; and-   generating a monitor signal corresponding to the power of the    optical signal using the gain control signal.

Advantageous Effects of Invention

The present invention can provide an optical reception apparatus and amonitor signal generating method, with which the power of an opticalsignal of a particular wavelength can be monitored.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an optical reception apparatusaccording to a first embodiment;

FIG. 2 is a circuit diagram showing one example of an amplifier circuitincluded in the optical reception apparatus according to the firstembodiment;

FIG. 3 is a block diagram showing an optical reception apparatusaccording to a second embodiment;

FIG. 4 is a diagram showing a 90-degree optical hybrid circuit includedin the optical reception apparatus according to the second embodiment;

FIG. 5 is a block diagram for describing details of an amplifier circuitincluded in the optical reception apparatus according to the secondembodiment;

FIG. 6 is a block diagram showing an optical reception apparatusaccording to a third embodiment;

FIG. 7 is a block diagram showing an optical reception apparatusaccording to a fourth embodiment;

FIG. 8 is a block diagram showing an optical reception apparatusaccording to a fifth embodiment; and

FIG. 9 is a block diagram showing an optical reception apparatusaccording to Comparative Example.

DESCRIPTION OF EMBODIMENTS First Embodiment

In the following, with reference to the drawings, a description will begiven of embodiments of the present invention. FIG. 1 is a block diagramshowing an optical reception apparatus 1 according to a firstembodiment. As shown in FIG. 1 , the optical reception apparatus 1according to the present embodiment includes a local oscillator (LO) 11,an optical mixer 12, a photoelectric converter 13, a variable gainamplifier 15, a gain control signal generating circuit 16, and a monitorsignal generating unit 17. Here, the variable gain amplifier 15 and thegain control signal generating circuit 16 configure an amplifier circuit14.

The optical reception apparatus 1 receives a multiplexed optical signal21 generated on the transmission apparatus side (not shown). Themultiplexed optical signal 21 is an optical signal in which opticalsignals being different in wavelength from each other are multiplexed.That is, the multiplexed optical signal 21 is an optical signal in whicha plurality of optical signals respectively having different wavelengthsλ₁, λ₂, ..., λ_(n) (n is an integer equal to or greater than 2) aremultiplexed. In the WDM communication, since such a multiplexed opticalsignal is used, large-volume information can be transmitted with asingle optical fiber.

The local oscillator 11 outputs local oscillation light 22 having aprescribed wavelength λ_(m) (m = 1 to n) to the optical mixer 12. Thatis, the local oscillator 11 outputs, to the optical mixer 12, the localoscillation light 22 of a wavelength λ_(m) corresponding to thewavelength of an optical signal to be extracted from the multiplexedoptical signal 21. For example, the local oscillator 11 includes awavelength variable laser, and is capable of changing the wavelengthλ_(m) of the local oscillation light 22 output from the local oscillator11 so as to correspond to the wavelength of an optical signal to beextracted from the multiplexed optical signal 21.

The optical mixer 12 receives the multiplexed optical signal 21 and thelocal oscillation light 22, and selects an optical signal 23corresponding to the wavelength of the local oscillation light 22 fromthe multiplexed optical signal 21. Then, the optical mixer 12 outputsthe selected optical signal 23 to the photoelectric converter 13. In thecoherent light transmission scheme, by causing the multiplexed opticalsignal 21 and the local oscillation light 22 to interfere with eachother and performing a coherent detection, an optical signalcorresponding to the wavelength λ_(m), of the local oscillation light 22can be selectively extracted from the multiplexed optical signal 21 inwhich a plurality of optical signals having the wavelengths λ₁, λ₂, ...,λ_(n) are multiplexed. Hence, by changing the wavelength λ_(m) of thelocal oscillation light 22 output from the local oscillator 11, anoptical signal to be extracted from the multiplexed optical signal 21can be arbitrarily selected.

The photoelectric converter 13 converts the optical signal 23 outputfrom the optical mixer 12 to an electric signal 24, and outputs theelectric signal 24 to the amplifier circuit 14. The photoelectricconverter 13 may be, for example, a photodiode.

The amplifier circuit 14 includes the variable gain amplifier 15 and thegain control signal generating circuit 16. The amplifier circuit 14configures an AGC (Automatic Gain Control) circuit.

The variable gain amplifier 15 amplifies the electric signal 24 outputfrom the photoelectric converter 13, and generates an output signal 25whose output amplitude is amplified to a certain level. At this time,the variable gain amplifier 15 adjusts the gain of the variable gainamplifier 15 in accordance with a gain control signal 26 generated bythe gain control signal generating circuit 16.

The gain control signal generating circuit 16 generates the gain controlsignal 26 for controlling the gain of the variable gain amplifier 15.For example, the gain control signal generating circuit 16 generates,based on the amplitude voltage of the output signal 25 output from thevariable gain amplifier 15 and a preset target voltage Vt, the gaincontrol signal 26 for feedback-controlling the variable gain amplifier15.

For example, the gain control signal generating circuit 16 generates thegain control signal 26 with which the amplitude voltage of the outputsignal 25 output from the variable gain amplifier 15 (in other words,the absolute value of the amplitude voltage of the output signal 25) andthe target voltage Vt become equal to each other. Specifically, when theamplitude voltage of the output signal 25 is greater than the targetvoltage Vt, the gain control signal generating circuit 16 generates thegain control signal 26 with which the gain of the variable gainamplifier 15 reduces. Conversely, when the amplitude voltage of theoutput signal 25 is smaller than the target voltage Vt, the gain controlsignal generating circuit 16 generates the gain control signal 26 withwhich the gain of the variable gain amplifier 15 increases.

Note that, in the present embodiment, as an amplifier circuit 14′ shownin FIG. 2 , a transimpedance amplifier 18 may be provided between thephotoelectric converter 13 and the variable gain amplifier 15. Forexample, when the electric signal 24 output from the photoelectricconverter 13 is a current signal, by providing the transimpedanceamplifier 18, the current signal can be converted to a voltage signal.

The monitor signal generating unit 17 generates a monitor signal 27using the gain control signal 26. The monitor signal 27 is a signalcorresponding to the power of the optical signal 23 output from theoptical mixer 12 (that is, the optical signal 23 selected from themultiplexed optical signal 21).

For example, when the variable gain amplifier 15 is configured such thatthe amplification factor of the variable gain amplifier 15 increases asthe signal voltage of the gain control signal 26 becomes higher, therelationship between the power of the optical signal 23 and the gaincontrol signal 26 is as follows. When the power of the optical signal 23is excessively small, the amplitude voltage of the electric signal 24also becomes small. In this case, since the difference between theamplitude voltage of the output signal 25 and the target voltage Vtbecomes great, the amplification factor of the variable gain amplifier15 must be increased. Hence, the signal voltage of the gain controlsignal 26 generated by the gain control signal generating circuit 16becomes high. On the other hand, when the power of the optical signal 23is close to the target value, the difference between the amplitudevoltage of the output signal 25 and the target voltage Vt becomes small.In this case, since the amplification factor of the variable gainamplifier 15 becomes small, the signal voltage of the gain controlsignal 26 generated by the gain control signal generating circuit 16becomes low.

Further, for example, when the variable gain amplifier 15 is configuredsuch that the amplification factor of the variable gain amplifier 15increases as the signal voltage of the gain control signal 26 becomeslower, the relationship between the power of the optical signal 23 andthe gain control signal 26 is as follows. When the power of the opticalsignal 23 is excessively small, the amplitude voltage of the electricsignal 24 also becomes small. In this case, since the difference betweenthe amplitude voltage of the output signal 25 and the target voltage Vtbecomes great, the amplification factor in the variable gain amplifier15 must be increased. Hence, the signal voltage of the gain controlsignal 26 generated by the gain control signal generating circuit 16becomes low. On the other hand, when the power of the optical signal 23is close to the target value, the difference between the amplitudevoltage of the output signal 25 and the target voltage Vt becomes small.In this case, since the amplification factor in the variable gainamplifier 15 becomes small, the signal voltage of the gain controlsignal 26 generated by the gain control signal generating circuit 16becomes high.

In this manner, the gain control signal 26 varies in accordance with thepower of the optical signal 23. The monitor signal generating unit 17can generate the monitor signal 27 corresponding to the power of theoptical signal 23 using the gain control signal 26 which varies in thismanner. For example, the monitor signal generating unit 17 may includean analog-digital converter circuit. In this case, the gain controlsignal 26 being an analog signal can be converted into a digital signal.

Note that, the optical reception apparatus 1 according to the presentembodiment may further include an analog-digital converter circuit (notshown) that converts the output signal 25 from an analog signal to adigital signal, and a digital signal processing circuit (not shown) thatprocesses the output signal converted into a digital signal.

In the coherent light transmission scheme disclosed in PatentLiteratures 1 and 2, a multiplexed optical signal transmitted from anoptical transmission apparatus is received using an optical receptionapparatus. For example, the power of a multiplexed optical signal inputto the optical reception apparatus can be monitored by branching themultiplexed optical signal input to the optical reception apparatus withan optical coupler or the like, and converting the branched multiplexedoptical signal to an electric signal with a monitor-purposephotoelectric converter.

FIG. 9 is a block diagram showing an optical reception apparatus 100according to Comparative Example. The optical reception apparatus 100shown in FIG. 9 includes an optical coupler 101, a photoelectricconverter 104, a local oscillator (LO) 111, an optical mixer 112, aphotoelectric converter 113, a variable gain amplifier 115, and a gaincontrol signal generating circuit 116. The variable gain amplifier 115and the gain control signal generating circuit 116 configure anamplifier circuit 114. Note that, in the optical reception apparatus 100shown in FIG. 9 , constituent elements identical to those of the opticalreception apparatus 1 shown in FIG. 1 are denoted by the referencenumerals in the 100 s.

In the optical reception apparatus 100 shown in FIG. 9 , the multiplexedoptical signal 121 input to the optical reception apparatus 100 isbranched by the optical coupler 101, and one multiplexed optical signal102 is input to the optical mixer 112 while other multiplexed opticalsignal 103 is input to the photoelectric converter 104. Then, byconverting the branched multiplexed optical signal 103 to an electricsignal by the photoelectric converter 104, the power of the multiplexedoptical signal 121 input to the optical reception apparatus 100 can bemonitored.

However, since the multiplexed optical signal 121 is an optical signalin which optical signals of a plurality of wavelengths are multiplexed,when the multiplexed optical signal 121 input to the optical receptionapparatus 100 is monitored, all the optical signals input to the opticalreception apparatus 100 are monitored. Hence, in this case, the power ofan optical signal of a particular wavelength 123 cannot be measuredsolely.

Accordingly, in the optical reception apparatus 1 according to thepresent embodiment, as shown in FIG. 1 , using the gain control signal26 for controlling the gain of the variable gain amplifier 15, themonitor signal 27 corresponding to the power of the optical signal 23output from the optical mixer 12 is generated. That is, the variablegain amplifier 15 amplifies solely the electric signal 24 correspondingto the optical signal 23 selected from the multiplexed optical signal21. Further, the gain control signal 26 is a signal for controlling thegain of the variable gain amplifier 15, and varies in accordance withthe power of the optical signal 23. Hence, by generating the monitorsignal 27 using the gain control signal 26, the power of the opticalsignal 23 can be monitored.

Further, with the optical reception apparatus 1 according to the presentembodiment, since the power of the optical signal 23 is monitored usingthe gain control signal 26, it is not necessary to provide the opticalcoupler 101 for branching the multiplexed optical signal or themonitor-purpose photoelectric converter 104 (see FIG. 9 ). Further, withthe optical reception apparatus 1 of the present embodiment, by causingthe multiplexed optical signal 21 and the local oscillation light 22 tointerfere with each other and performing a coherent detection, theoptical signal 23 corresponding to the wavelength of the localoscillation light 22 is selectively extracted from the multiplexedoptical signal 21. Hence, it is not necessary to provide an arrayedwaveguide grating (AWG) or an optical filter for extracting an opticalsignal from a multiplexed optical signal. Accordingly, the opticalreception apparatus can be reduced in size, and the manufacturing costsof the optical reception apparatus can be reduced.

By the invention according to the present embodiment described above,the optical reception apparatus and the monitor signal generating methodwith which the power of an optical signal of a particular wavelength canbe monitored can be provided.

Second Embodiment

Next, a description will be given of a second embodiment of the presentinvention. In the present embodiment, a description will be given of thecase where the optical reception apparatus described in the firstembodiment is applied to the dual polarization quadrature phase shiftkeying (DP-QPSK) scheme.

FIG. 3 is a block diagram showing an optical reception apparatus 2according to the present embodiment. As shown in FIG. 3 , the opticalreception apparatus 2 according to the present embodiment includes alocal oscillator (LO) 31, a polarization beam splitter (PBS) 32, a90-degree optical hybrid circuit 34_1, 34_2, a photoelectric converter35, amplifier circuits 36_1 to 36_4, monitor signal generating units37_1 to 37_4, analog-digital converter circuits 38_1 to 38_4, and adigital signal processing circuit 39.

The optical reception apparatus 2 receives a multiplexed optical signal51 generated on the transmission apparatus side (not shown). Themultiplexed optical signal 51 is an optical signal in which opticalsignals being different in wavelength from each other are multiplexed.Further, in the present embodiment, in the multiplexed optical signal51, X polarized light (first polarized light) and Y polarized light(second polarized light) being orthogonal to each other are multiplexed.The X polarized light and the Y polarized light are modulatedindependently of each other, and capable of independently transmittinginformation. Further, the X polarized light and the Y polarized lightare each modulated by four different phases.

The polarization beam splitter 32 receives the multiplexed opticalsignal 51, and splits the multiplexed optical signal 51 into the Xpolarized light 52 and the Y polarized light 53 being orthogonal to eachother. Then, the polarization beam splitter 32 outputs the split Xpolarized light 52 to the 90-degree optical hybrid circuit 34_1 (a firstoptical hybrid circuit), and outputs the split Y polarized light to the90-degree optical hybrid circuit 34_2 (a second optical hybrid circuit).

The local oscillator 31 outputs local oscillation light 54 having aprescribed wavelength to each of the 90-degree optical hybrid circuits34_1, 34_2. That is, the local oscillator 31 outputs, to 90-degreeoptical hybrid circuits 34_1, 34_2, the local oscillation light 54having the wavelength corresponding to the wavelength of an opticalsignal to be extracted from the multiplexed optical signal 51. Forexample, the local oscillator 31 is configured to include a wavelengthvariable laser, and capable of varying the wavelength of the localoscillation light 54 output from the local oscillator 31 so as tocorrespond to the wavelength of the optical signal to be extracted fromthe multiplexed optical signal 51.

The 90-degree optical hybrid circuit 34_1 includes an optical mixer (afirst optical mixer). The 90-degree optical hybrid circuit 34_1 receivesthe X polarized light 52 and the local oscillation light 54 and causesthe X polarized light 52 and the local oscillation light 54 to interferewith each other, thereby separating an optical signal corresponding tothe wavelength of the local oscillation light 31 from the X polarizedlight 52. Further, the 90-degree optical hybrid circuit 34_1 splits theX polarized light 52 into an in-phase component (the I component) and aquadrature component (the Q component). Then, the 90-degree opticalhybrid circuit 34_1 outputs two optical signals included in the in-phasecomponent as first differential signals, and outputs two optical signalsincluded in the quadrature component as second differential signals.

FIG. 4 is a diagram showing one example of the 90-degree optical hybridcircuit 34_1. As shown in FIG. 4 , the 90-degree optical hybrid circuit34_1 includes optical couplers 61_1 to 61_3, 62_1 to 62_3, a π/2 phaseshifter 63, π phase shifters 64_1, 64_2, and optical mixers 65_1 to 65_4(the first optical mixer).

The X polarized light 52 input to the 90-degree optical hybrid circuit34_1 is branched by the optical couplers 61_1 to 61_3, and introduced tothe optical mixers 65_1 to 65_4. The local oscillation light 54 input tothe 90-degree optical hybrid circuit 34_1 is branched by the opticalcoupler 62_1 and the optical coupler 62_3, and thereafter introduced tothe optical mixer 65_1. The local oscillation light 54 input to the90-degree optical hybrid circuit 34_1 is branched by the optical coupler62_1 and the optical coupler 62_3, and thereafter has its phase shiftedby π by the π phase shifter 64_1, to be introduced to the optical mixer65_2.

The local oscillation light 54 input to the 90-degree optical hybridcircuit 34_1 is branched by the optical coupler 62_1, and thereafter hasits phase shifted by π/2 by the π/2 phase shifter 63. The localoscillation light 54 is further branched by the optical coupler 62_2 andthereafter introduced to optical mixer 65_3. The local oscillation light54 input to the 90-degree optical hybrid circuit 34_1 is branched by theoptical coupler 62_1, and thereafter has its phase shifted by π/2 by theπ/2 phase shifter 63. The local oscillation light 54 is further branchedby the optical coupler 62_2, and thereafter has its phase shifted by πby the π phase shifter 64_2, to be introduced to the optical mixer 65_4.

That is, the optical mixer 65_1 receives the local oscillation light 54which is in-phase; the optical mixer 65_2 receives the local oscillationlight 54 which is out of phase by π; the optical mixer 65_3 receives thelocal oscillation light 54 which is out of phase by π/2; and the opticalmixer 65_4 receives the local oscillation light 54 which is out of phaseby 3π/2.

Therefore, the optical mixer 65_1 outputs an optical signal Ip_1 whichis in-phase; the optical mixer 65_2 outputs an optical signal In_1 whichis out of phase by π; the optical mixer 65_3 outputs an optical signalQp_1 which is out of phase by π/2; and the optical mixer 65_4 outputs anoptical signal Qn_1 which is out of phase by 3π/2. The optical signalIp_1 and the optical signal In_1 are output as the first differentialsignals (differential signals of the in-phase component), and theoptical signal Qp_1 and the optical signal Qn_1 are output as the seconddifferential signals (the differential signals of the quadraturecomponent).

The 90-degree optical hybrid circuit 34_2 operates similarly to the90-degree optical hybrid circuit 34_1. That is, the 90-degree opticalhybrid circuit 34_2 includes an optical mixer (a second optical mixer).The 90-degree optical hybrid circuit 34_2 receives the Y polarized light53 and the local oscillation light 54 and causes the Y polarized light53 and the local oscillation light 54 to interfere with each other,thereby separating an optical signal corresponding to the wavelength ofthe local oscillation light 31 from the Y polarized light 53. Further,the 90-degree optical hybrid circuit 34_2 splits the Y polarized light53 into the in-phase component (the I component) and the quadraturecomponent (the Q component). Then, the 90-degree optical hybrid circuit34_2 outputs two optical signals (Ip_2, In_2) included in the in-phasecomponent as third differential signals, and outputs two optical signals(Qp_2, Qn_2) included in the quadrature component as fourth differentialsignals.

As shown in FIG. 3 , the optical signals Ip_1, In_1 (the firstdifferential signals), the optical signals Qp_1, Qn_1 (the seconddifferential signals), the optical signals Ip_2, In_2 (the thirddifferential signals), and the optical signals Qp_2, Qn_2 (the fourthdifferential signal) output from the 90-degree optical hybrid circuits34_1, 34_2 are respectively converted by photoelectric converters PD#1to PD#8 to electric signals.

The amplifier circuit 36_1 amplifies the electric signals correspondingto the first differential signals (Ip_1, In_1) output from thephotoelectric converters PD#1, PD#2, and generates output signals whoseoutput amplitude is amplified to a certain level. The generated outputsignals are output to the analog-digital converter circuit 38_1. Theanalog-digital converter circuit 38_1 converts the output signals fromanalog signals to digital signals, and outputs the digital signals tothe digital signal processing circuit 39. The monitor signal generatingunit 37_1 generates a monitor signal 60_1 corresponding to the firstdifferential signals (Ip_1, In_1).

The amplifier circuit 36_2 amplifies the electric signals correspondingto the second differential signals (Qp_1, Qn_1) output from thephotoelectric converters PD#3, PD#4, and generates output signals whoseoutput amplitude is amplified to a certain level. The generated outputsignals are output to the analog-digital converter circuit 38_2. Theanalog-digital converter circuit 38_2 converts the output signals fromanalog signals to digital signals, and outputs the digital signals tothe digital signal processing circuit 39. The monitor signal generatingunit 37_2 generates a monitor signal 60_2 corresponding to the seconddifferential signals (Qp_1, Qn_1).

The amplifier circuit 36_3 amplifies the electric signals correspondingto the third differential signals (Ip_2, In_2) output from thephotoelectric converters PD#5, PD#6, and generates output signals whoseoutput amplitude is amplified to a certain level. The generated outputsignals are output to the analog-digital converter circuit 38_3. Theanalog-digital converter circuit 38_3 converts the output signals fromanalog signals to digital signals, and outputs the digital signals tothe digital signal processing circuit 39. The monitor signal generatingunit 37_3 generates a monitor signal 60_3 corresponding to the thirddifferential signals (Ip_2, In_2).

The amplifier circuit 36_4 amplifies the electric signals correspondingto the fourth differential signals (Qp_2, Qn_2) output from thephotoelectric converters PD#7, PD#8, and generates output signals whoseoutput amplitude is amplified to a certain level. The generated outputsignals are output to the analog-digital converter circuit 38_4. Theanalog-digital converter circuit 38_4 converts the output signals fromanalog signals to digital signals, and outputs the digital signals tothe digital signal processing circuit 39. The monitor signal generatingunit 37_4 generates a monitor signal 60_4 corresponding to the fourthdifferential signals (Qp_2, Qn_2).

FIG. 5 is a block diagram for describing details of the amplifiercircuit 36_1 included in the optical reception apparatus 2 according tothe present embodiment. While a description will be given of theamplifier circuit 36_1 in the following, the same holds true for otheramplifier circuits 36_2 to 36_4.

The amplifier circuit 36_1 includes a transimpedance amplifier 42, avariable gain amplifier 43, and a gain control signal generating circuit44. The amplifier circuit 36_1 configures an AGC circuit. Thedifferential signals (Ip_1, In_1) output from the 90-degree opticalhybrid circuit 34_1 are converted to differential signals 56_1, 56_2 bythe photoelectric converters PD#1, PD#2, and supplied to thetransimpedance amplifier 42. The transimpedance amplifier 42 convertsthe differential signals 56_1, 56_2 from current signals to voltagesignals, and outputs differential signals 57_1, 57_2 to the variablegain amplifier 43. Note that, the transimpedance amplifier 42 may beomitted.

The variable gain amplifier 43 amplifies the differential signals 57_1,57_2, and generates differential output signals 58_1, 58_2 whose outputamplitude is amplified to a certain level. At this time, the variablegain amplifier 43 adjusts the gain of the variable gain amplifier 43 inaccordance with a gain control signal 59 generated by the gain controlsignal generating circuit 44.

The gain control signal generating circuit 44 generates the gain controlsignal 59 for controlling the gain of the variable gain amplifier 43.For example, the gain control signal generating circuit 44 generates thegain control signal 59 for feedback-controlling the variable gainamplifier 43, based on the amplitude voltage of the differential outputsignals 58_1, 58_2 output from the variable gain amplifier 43 and thepreset target voltage Vt.

For example, the gain control signal generating circuit 44 generates thegain control signal 59 with which the amplitude voltage of thedifferential output signals 58_1, 58_2 output from the variable gainamplifier 43 (in other words, the absolute value of the amplitudevoltage of the differential output signals 58_1, 58_2) and the targetvoltage Vt become equal to each other. Specifically, when the amplitudevoltage of the differential output signals 58_1, 58_2 is higher than thetarget voltage Vt, the gain control signal generating circuit 44generates the gain control signal 59 with which the gain of the variablegain amplifier 43 reduces. Conversely, when the amplitude voltage of thedifferential output signals 58_1, 58_2 is lower than the target voltageVt, the gain control signal generating circuit 44 generates the gaincontrol signal 59 with which the gain of the variable gain amplifier 43increases.

The monitor signal generating unit 37_1 generates the monitor signal60_1 using the gain control signal 59. The monitor signal 60_1 is asignal corresponding to the power of the differential signals 55_1, 55_2(Ip_1, In_1) output from the 90-degree optical hybrid circuit 34_1.

That is, as described in the first embodiment, the gain control signal59 varies in accordance with the power of the differential signals 55_1,55_2 (Ip_1, In_1). The monitor signal generating unit 37_1 can generatethe monitor signal 60_1 corresponding to the power of the differentialsignals 55_1, 55_2 (Ip_1, In_1) using the gain control signal 59 varyingin this manner. For example, the monitor signal generating unit 37_1 mayinclude an analog-digital converter circuit. In this case, the gaincontrol signal 59 being an analog signal can be converted to a digitalsignal.

In the optical reception apparatus 2 according to the present embodimentalso, the power of the optical signals output from the 90-degree opticalhybrid circuits 34_1, 34_2 is monitored using the gain control signal 59for controlling the gain of the variable gain amplifier 43. Hence, thepower of an optical signal of a particular wavelength can be monitored.

Note that, while the description has been given of the case where fourmonitor signal generating units 37_1 to 37_4 are included with referenceto FIG. 3 , at least one monitor signal generating unit will suffice.That is, the monitor signal generating unit may be provided for only thedifferential signals that must be monitored, out of the firstdifferential signals (Ip_1, In_1), the second differential signals(Qp_1, Qn_1), the third differential signals (Ip_2, In_2), and thefourth differential signals (Qp_2, Qn_2).

Further, in the foregoing, the description has been given of the casewhere the multiplexed optical signal 51 includes the X polarized lightand the Y polarized light. However, the invention according to thepresent embodiment may be applied also to the quadrature phase shiftkeying (QPSK) scheme in which no polarized light is used. In this case,the polarization beam splitter 32, the 90-degree optical hybrid circuit34_2, the photoelectric converters PD#5 to PD#8, the amplifier circuits36_3, 36_4, the monitor signal generating units 37_3, 37_4, and theanalog-digital converter circuits 38_3, 38_4 can be omitted. In the casewhere the invention according to the present embodiment is applied tothe quadrature phase shift keying scheme, the 90-degree optical hybridcircuit 34_1 receives a multiplexed optical signal and local oscillationlight, and the 90-degree optical hybrid circuit 34_1 outputs fouroptical signals Ip_1, In_1, Qp_1, Qn_1 (in other words, two types ofdifferential signals). The optical signals Ip_1, In_1, Qp_1, and Qn_1are processed similarly to the manner described above.

Third Embodiment

Next, a description will be given of a third embodiment of the presentinvention. FIG. 6 is a block diagram showing an optical receptionapparatus according to the third embodiment. The optical receptionapparatus 3 according to the third embodiment is different from theoptical reception apparatus 1 described in the first embodiment in thatthe power of the local oscillation light 22 output from a localoscillator 71 is controlled in accordance with the monitor signal 27generated by the monitor signal generating unit 17. Other configurationis similar to that of the optical reception apparatus 1 described in thefirst embodiment, and therefore identical constituent elements aredenoted by identical reference numerals, and repetitive descriptions areomitted.

As shown in FIG. 6 , the optical reception apparatus 3 according to thepresent embodiment includes the local oscillator (LO) 71, the opticalmixer 12, the photoelectric converter 13, the variable gain amplifier15, the gain control signal generating circuit 16, the monitor signalgenerating unit 17, and a local oscillation light control unit 72.

The optical mixer 12 receives the multiplexed optical signal 21 and thelocal oscillation light 22, and selects the optical signal 23corresponding to the wavelength of the local oscillation light 22 fromthe multiplexed optical signal 21. Then, the optical mixer 12 outputsthe selected optical signal 23 to the photoelectric converter 13. Atthis time, the optical mixer 12 causes the multiplexed optical signal 21and the local oscillation light 22 to interfere with each other andperforms a coherent detection, thereby selectively extracting theoptical signal corresponding to the wavelength λ_(m) of the localoscillation light 22 from the multiplexed optical signal 21. Hence, inorder to properly extract the optical signal 23 of a particularwavelength from the multiplexed optical signal 21, it is necessary toadjust the power of the local oscillation light 22 input to the opticalmixer 12 to a proper value.

Accordingly, with the optical reception apparatus 3 according to thepresent embodiment, the power of the local oscillation light 22 outputfrom the local oscillator 71 is controlled in accordance with themonitor signal 27 generated by the monitor signal generating unit 17.That is, the local oscillation light control unit 72 generates a controlsignal 73 for controlling the local oscillator 71 in accordance with themonitor signal 27, and outputs the control signal 73 to the localoscillator 71. The local oscillator 71 adjusts the power of the localoscillation light 22 in accordance with the control signal 73.

For example, when the power of the local oscillation light 22 isexcessively small, the power of the optical signal 23 output from theoptical mixer 12 also becomes small. At this time, since the monitorsignal 27 indicates that the power of the optical signal 23 isexcessively small, the local oscillation light control unit 72 controlsthe local oscillator 71 to increase the power of the local oscillationlight 22.

Further, for example when the power of the local oscillation light 22 isexcessively great, the power of the optical signal 23 output from theoptical mixer 12 also becomes great. At this time, since the monitorsignal 27 indicates that the power of the optical signal 23 isexcessively great, the local oscillation light control unit 72 controlsthe local oscillator 71 to reduce the power of the local oscillationlight 22.

For example, the local oscillation light control unit 72 may control thepower of the local oscillation light 22 such that the value of themonitor signal 27 (that is, the power value of the optical signal 23)attains a prescribed value. Here, such a prescribed value can bearbitrarily determined.

In this manner, since the optical reception apparatus 3 according to thepresent embodiment can control the power of the local oscillation light22 in accordance with the monitor signal 27, the optical signal 23having prescribed power can be extracted from the multiplexed opticalsignal 21.

Fourth Embodiment

Next, a description will be given of a fourth embodiment of the presentinvention. FIG. 7 is a block diagram showing an optical receptionapparatus according to the fourth embodiment. The optical receptionapparatus 4 according to the fourth embodiment is different from theoptical reception apparatus 1 described in the first embodiment in thatthe power of a multiplexed optical signal 84 supplied to the opticalmixer 12 is adjusted in accordance with the monitor signal 27 generatedby the monitor signal generating unit 17. Other configuration is similarto that of the optical reception apparatus 1 described in the firstembodiment, and therefore identical constituent elements are denoted byidentical reference numerals, and repetitive descriptions are omitted.

As shown in FIG. 7 , the optical reception apparatus 4 according to thepresent embodiment includes the local oscillator (LO) 11, a multiplexedoptical signal adjusting unit 81, the optical mixer 12, thephotoelectric converter 13, the variable gain amplifier 15, the gaincontrol signal generating circuit 16, the monitor signal generating unit17, and a multiplexed optical signal control unit 82.

The multiplexed optical signal adjusting unit 81 adjusts the power ofthe multiplexed optical signal 21, and outputs the adjusted multiplexedoptical signal 84 to the optical mixer 12. The multiplexed opticalsignal control unit 82 controls the multiplexed optical signal adjustingunit 81 in accordance with the monitor signal 27. The multiplexedoptical signal adjusting unit 81 can be configured, for example, usingan attenuator (attenuator) that attenuates the multiplexed opticalsignal 21 in accordance with a control signal 83 output from themultiplexed optical signal control unit 82.

The optical mixer 12 receives the multiplexed optical signal 84 and thelocal oscillation light 22, and selects the optical signal 23corresponding to the wavelength of the local oscillation light 22 fromthe multiplexed optical signal 84. Then, the optical mixer 12 outputsthe selected optical signal 23 to the photoelectric converter 13. Atthis time, the optical mixer 12 causes the multiplexed optical signal 84and the local oscillation light 22 to interfere with each other andperforms a coherent detection, thereby selectively extracting theoptical signal corresponding to the wavelength λ_(m) of the localoscillation light 22 from the multiplexed optical signal 21. Hence, inorder to properly extract the optical signal 23 of a particularwavelength from the multiplexed optical signal 84, it is necessary toadjust the power of the multiplexed optical signal 84 input to theoptical mixer 12 to a proper value.

Accordingly, with the optical reception apparatus 4 according to thepresent embodiment, the power of the multiplexed optical signal 84 inputto the optical mixer 12 is adjusted in accordance with the monitorsignal 27 generated by the monitor signal generating unit 17. Themultiplexed optical signal control unit 82 generates the control signal83 for controlling the multiplexed optical signal adjusting unit 81 inaccordance with the monitor signal 27, and outputs the control signal 83to the multiplexed optical signal adjusting unit 81. The multiplexedoptical signal adjusting unit 81 adjusts the power of the multiplexedoptical signal 21 in accordance with the control signal 83, and outputsthe adjusted multiplexed optical signal 84 to the optical mixer 12.

For example, when the power of the multiplexed optical signal 84 isexcessively great, the power of the optical signal 23 output from theoptical mixer 12 also becomes great. At this time, since the monitorsignal 27 indicates that the power of the optical signal 23 isexcessively great, the multiplexed optical signal control unit 82controls the multiplexed optical signal adjusting unit 81 to reduce thepower of the multiplexed optical signal 84 input to the optical mixer12.

Further, for example when the power of the multiplexed optical signal 84is excessively small, the power of the optical signal 23 output from theoptical mixer 12 also becomes small. At this time, since the monitorsignal 27 indicates that the power of the optical signal 23 isexcessively small, the multiplexed optical signal control unit 82controls the multiplexed optical signal adjusting unit 81 to increasethe power of the multiplexed optical signal 84 input to the opticalmixer 12.

For example, the multiplexed optical signal control unit 82 may controlthe power of the multiplexed optical signal 84 such that the value ofthe monitor signal 27 (that is, the power value of the optical signal23) attains a prescribed value. Here, such a prescribed value can bearbitrarily determined.

In this manner, since the optical reception apparatus 4 according to thepresent embodiment can control the power of the multiplexed opticalsignal 84 input to the optical mixer 12 in accordance with the monitorsignal 27, the optical signal 23 having prescribed power can beextracted.

Fifth Embodiment

Next, a description will be given of a fifth embodiment of the presentinvention. FIG. 8 is a block diagram showing an optical receptionapparatus according to the fifth embodiment. The optical receptionapparatus 5 according to the fifth embodiment has a configuration inwhich the optical reception apparatus 3 according to the thirdembodiment and the optical reception apparatus 4 according to the fourthembodiment are combined.

That is, the optical reception apparatus 5 according to the presentembodiment controls the power of the local oscillation light 22 inaccordance with the monitor signal 27 generated by the monitor signalgenerating unit 17, and further adjusts the power of the multiplexedoptical signal 84 in accordance with the monitor signal 27.

As shown in FIG. 8 , the optical reception apparatus 5 according to thepresent embodiment includes the local oscillator (LO) 71, the opticalmixer 12, the photoelectric converter 13, the variable gain amplifier15, the gain control signal generating circuit 16, the monitor signalgenerating unit 17, the local oscillation light control unit 72, themultiplexed optical signal adjusting unit 81, and the multiplexedoptical signal control unit 82. Note that, these constituent elementsare similar to those in the first, third and fourth embodiments, andtherefore identical constituent elements are denoted by identicalreference numerals, and repetitive descriptions are omitted.

With the optical reception apparatus 5 according to the presentembodiment, the power of the local oscillation light 22 can becontrolled in accordance with the monitor signal 27. Further, the powerof the multiplexed optical signal 84 can be adjusted in accordance withthe monitor signal 27. Hence, since the power of the local oscillationlight 22 and the power of the multiplexed optical signal 84 can becontrolled independently of each other, as compared to the opticalreception apparatus according to the third and fourth embodiments, thepower of the optical signal 23 output from the optical mixer 12 can beprecisely adjusted.

Note that, in the fourth and fifth embodiments, the description has beengiven of the case where the power of the multiplexed optical signal 84input to the optical mixer 12 is adjusted using the multiplexed opticalsignal adjusting unit 81 and the multiplexed optical signal control unit82. However, the power of the multiplexed optical signal 21 may beadjusted on the transmission apparatus side transmitting the multiplexedoptical signal 21. In this case, the monitor signal 27 must betransmitted to the transmission apparatus side.

Further, the invention described in the third to fifth embodiments isalso applicable to an optical reception apparatus of the dualpolarization quadrature phase shift keying (DP-QPSK) scheme described inthe second embodiment.

In the foregoing, though the present invention has been described withreference to the embodiments, the present invention is not limitedthereby. Various modifications that can be understood by a personskilled in the art can be made to the configuration or details of thepresent invention within the scope of the invention.

The present application claims priority based on Japanese PatentApplication No. 2013-145238 filed on Jul. 11, 2013, the entiredisclosure of which is incorporated herein by reference.

Reference Signs List

-   1, 2, 3, 4, 5 OPTICAL RECEPTION APPARATUS-   11 LOCAL OSCILLATOR (LO)-   12 OPTICAL MIXER-   13 PHOTOELECTRIC CONVERTER-   14 AMPLIFIER CIRCUIT-   15 VARIABLE GAIN AMPLIFIER-   16 GAIN CONTROL SIGNAL GENERATING CIRCUIT-   17 MONITOR SIGNAL GENERATING UNIT-   18 TRANSIMPEDANCE AMPLIFIER-   21 MULTIPLEXED OPTICAL SIGNAL-   22 LOCAL OSCILLATION LIGHT-   23 OPTICAL SIGNAL-   24 ELECTRIC SIGNAL-   25 OUTPUT SIGNAL-   26 GAIN CONTROL SIGNAL-   27 MONITOR SIGNAL-   31 LOCAL OSCILLATOR (LO)-   32 POLARIZATION BEAM SPLITTER-   34_1, 34_2 90-DEGREE OPTICAL HYBRID CIRCUIT-   35 PHOTOELECTRIC CONVERTER-   36_1 to 36 4AMPLIFIER CIRCUIT-   37_1 to 37_4 MONITOR SIGNAL GENERATING UNIT-   38_1 to 38_ 4 ANALOG-DIGITAL CONVERTER CIRCUIT-   39 DIGITAL SIGNAL PROCESSING CIRCUIT-   61_1 to 61_3, 62_1 to 62_3 OPTICAL COUPLER-   63 π/2 PHASE SHIFTER-   64_1, 64_2 π PHASE SHIFTER-   65_1 to 65_4 OPTICAL MIXER-   71 LOCAL OSCILLATOR (LO)-   72 LOCAL OSCILLATION LIGHT CONTROL UNIT-   81 MULTIPLEXED OPTICAL SIGNAL ADJUSTING UNIT-   82 MULTIPLEXED OPTICAL SIGNAL CONTROL UNIT

1. An optical receiver comprising: a variable optical attenuatorconfigured to attenuate power of an input multiplexed optical signalcomprising a plurality of optical signals; an optical hybrid configuredto receive the plurality of attenuated optical signals and a localoscillation light and output an optical signal; a photo detectorconfigured to convert the optical signal from the optical hybrid to anelectrical signal; an amplifier configured to amplify the electricalsignal with a gain; and a processor configured to output, to theamplifier, a gain control signal corresponding to the gain, wherein whenan amplitude of the gain control signal increases, the variable opticalattenuator decreases attenuation of the power of the input multiplexedoptical signal.
 2. The optical receiver according to claim 1, whereinthe amplifier has an automatic gain control.
 3. The optical receiveraccording to claim 2, wherein the variable optical attenuator increasesor decreases attenuation of the input multiplexed optical signal by theautomatic gain control.
 4. The optical receiver according to claim 2,wherein the automatic gain control includes a target value for amplitudeof the amplified electrical signal.
 5. The optical receiver according toclaim 1, wherein the amplifier comprises a transimpedance amplifier. 6.The optical receiver according to claim 1, further comprising: ananalog-digital converter configured to convert the amplified electricalsignal to a digital signal; and a digital signal processor configured toprocess the digital signal.
 7. The optical receiver according to claim1, wherein the variable optical attenuator attenuates the power of theinput multiplexed optical signal depending on a reference value forcalibration of the gain.
 8. The optical receiver according to claim 1,wherein a local oscillation light is output from a local oscillator. 9.An optical communication method comprising: attenuating power of aninput multiplexed optical signal comprising a plurality of opticalsignals; receiving the plurality of attenuated optical signals and thelocal oscillation light and output an optical signal; converting thereceived optical signal to an electrical signal; amplifying theelectrical signal with a gain; generating a gain control signalcorresponding to the gain; and when an amplitude of the gain controlsignal increases, decreasing attenuation of the power of the inputmultiplexed optical signal.
 10. The optical communication methodaccording to claim 9, further comprising of adjusting amplification ofthe electrical signal by an automatic gain control.
 11. The opticalcommunication method according to claim 10, further comprising ofincreasing or decreasing attenuation of the input multiplexed opticalsignal by the auto gain control.
 12. The optical communication methodaccording to claim 10, wherein the automatic gain control includes atarget value for amplitude of the amplified electrical signal.
 13. Theoptical communication method according to claim 9, further comprising:converting the amplified electrical signal to a digital signal; andprocessing the digital signal.
 14. The optical communication methodaccording to claim 9, further comprising of attenuating the power of theinput multiplexed optical signal depending on a reference value forcalibration of the gain.
 15. The optical communication method accordingto claim 9, further comprising of outputting a local oscillation lightfrom a local oscillator.
 16. The optical receiver according to claim 8,further comprising the local oscillator that outputs the localoscillation light.
 17. The optical receiver according to claim 16,wherein the local oscillator adjusts power of the local oscillationlight depending on the gain.
 18. The optical receiver according to claim1, further comprising a monitor configured to monitor the power of theinput multiplexed optical signal.
 19. The optical receiver according toclaim 1, wherein the optical hybrid is configured to output a firstoptical signal and a second optical signal included in an in-phasecomponent and a third optical signal and a fourth optical signalincluded in a quadrature component based on the plurality of attenuatedoptical signals and the local oscillation light; the photo detectorcomprises a first photo diode configured to convert the first opticalsignal into a first electrical signal; a second photo diode configuredto convert the second optical signal into a second electrical signal; athird photo diode configured to convert the third optical signal into athird electrical signal; and a fourth photo diode configured to convertthe fourth optical signal into a fourth electrical signal; and theamplifier comprises a first trans-impedance-amplifier configured toamplify a first differential electrical signal between the firstelectrical signal and the second electrical signal, and a secondtrans-impedance-amplifier configured to amplify a second differentialelectrical signal between the first electrical signal and the secondelectrical signal.
 20. The optical receiver according to claim 19,further comprising a first power monitor configured to monitor power ofthe first differential electrical signal output from the firsttrans-impedance-amplifier; and a second power monitor configured tomonitor power of the second differential electrical signal output fromthe second trans-impedance-amplifier.
 21. The optical receiver accordingto claim 1, further comprising a first optical hybrid being the opticalhybrid; a second optical hybrid; and a polarization beam splitterconfigured to split the plurality of attenuated optical signals into afirst polarized light and the second polarized light; wherein the firstoptical hybrid is configured to interfere the first polarized light andthe local oscillation light; and the second optical hybrid is configuredto interfere the second polarized light and the local oscillation light.22. The optical receiver according to claim 21, wherein the firstoptical hybrid is configured to output a first optical signal and asecond optical signal included in an in-phase component and a thirdoptical signal and a fourth optical signal included in a quadraturecomponent based on the plurality of attenuated optical signals and thelocal oscillation light; the second optical hybrid is configured tooutput a fifth optical signal and a sixth optical signal included in thein-phase component and a seventh optical signal and an eighth opticalsignal included in the quadrature component based on the plurality ofattenuated optical signals and the local oscillation light; the photodetector comprises a first photo diode configured to convert the firstoptical signal into a first electrical signal; a second photo diodeconfigured to convert the second optical signal into a second electricalsignal; a third photo diode configured to convert the third opticalsignal into a third electrical signal; and a fourth photo diodeconfigured to convert the fourth optical signal into a fourth electricalsignal; a fifth photo diode configured to convert the fifth opticalsignal into a fifth electrical signal; a sixth photo diode configured toconvert the sixth optical signal into a sixth electrical signal; aseventh photo diode configured to convert the seventh optical signalinto a seventh electrical signal; and an eighth photo diode configuredto convert the eighth optical signal into an eighth electrical signal;and the amplifier comprises a first trans-impedance-amplifier configuredto amplify a first differential signal between the first electricalsignal and the second electrical signal, and a secondtrans-impedance-amplifier configured to amplify a second differentialsignal between the first electrical signal and the second electricalsignal a third trans-impedance-amplifier configured to amplify a thirddifferential signal between the fifth electrical signal and the sixthelectrical signal, and a fourth trans-impedance-amplifier configured toamplify a fourth differential signal between the seventh electricalsignal and the eighth electrical signal.
 23. The optical receiveraccording to claim 22, further comprising a first monitor signalgenerator configured to generate a first monitor signal corresponding tothe first differential signal from the first trans-impedance-amplifier;a second monitor signal generator configured to generate a secondmonitor signal corresponding to the second differential signal from thesecond trans-impedance-amplifier; a third monitor signal generatorconfigured to generate a third monitor signal corresponding to the thirddifferential signal from the third trans-impedance-amplifier; a fourthmonitor signal generator configured to generate a fourth monitor signalcorresponding to the fourth differential signal from the fourthtrans-impedance-amplifier.
 24. The optical receiver according to claim1, wherein when the amplitude of the gain control signal decreases, thevariable optical attenuator increases attenuation of the power of theinput multiplexed optical signal.
 25. The optical receiver according toclaim 1, further comprising an analog-digital converter configured toconvert the gain control signal into a digital signal.
 26. The opticalcommunication method according to claim 9, further comprising:outputting a local oscillation light.
 27. The optical communicationmethod according to claim 26, wherein the local oscillation light isoutput from a local oscillator.
 28. The optical communication methodaccording to claim 9, further comprising monitoring the power of theinput multiplexed optical signal by a monitor.
 29. The opticalcommunication method according to claim 9, wherein an optical hybrid isconfigured to output a first optical signal and a second optical signalincluded in an in-phase component and a third optical signal and afourth optical signal included in a quadrature component based on theplurality of attenuated optical signals and the local oscillation light;a photo detector comprises a first photo diode configured to convert thefirst optical signal into a first electrical signal; a second photodiode configured to convert the second optical signal into a secondelectrical signal; a third photo diode configured to convert the thirdoptical signal into a third electrical signal; and a fourth photo diodeconfigured to convert the fourth optical signal into a fourth electricalsignal; and an amplifier comprises a first trans-impedance-amplifierconfigured to amplify a first differential electrical signal between thefirst electrical signal and the second electrical signal, and a secondtrans-impedance-amplifier configured to amplify a second differentialelectrical signal between the first electrical signal and the secondelectrical signal.
 30. The optical communication method according toclaim 29, wherein a first power monitor is configured to monitor powerof the first differential electrical signal output from the firsttrans-impedance-amplifier; and a second power monitor is configured tomonitor power of the second differential electrical signal output fromthe second transimpedance-amplifier.
 31. The optical communicationmethod according to claim 9, wherein an optical hybrid includes a firstoptical hybrid being the optical hybrid; and a second optical hybrid;and a polarization beam splitter configured to split the plurality ofattenuated optical signals into a first polarized light and the secondpolarized light, wherein: the first optical hybrid is configured tointerfere the first polarized light and the local oscillation light; andthe second optical hybrid is configured to interfere the secondpolarized light and the local oscillation light.
 32. The opticalcommunication method according to claim 31, wherein the first opticalhybrid is configured to output a first optical signal and a secondoptical signal included in an in-phase component and a third opticalsignal and a fourth optical signal included in a quadrature componentbased on the plurality of attenuated optical signals and the localoscillation light; the second optical hybrid is configured to output afifth optical signal and a sixth optical signal included in the in-phasecomponent and a seventh optical signal and an eighth optical signalincluded in the quadrature component based on the plurality ofattenuated optical signals and the local oscillation light; a photodetector comprises a first photo diode configured to convert the firstoptical signal into a first electrical signal; a second photo diodeconfigured to convert the second optical signal into a second electricalsignal; a third photo diode configured to convert the third opticalsignal into a third electrical signal; and a fourth photo diodeconfigured to convert the fourth optical signal into a fourth electricalsignal; a fifth photo diode configured to convert the fifth opticalsignal into a fifth electrical signal; a sixth photo diode configured toconvert the sixth optical signal into a sixth electrical signal; aseventh photo diode configured to convert the seventh optical signalinto a seventh electrical signal; and an eighth photo diode configuredto convert the eighth optical signal into an eighth electrical signal;and an amplifier comprises a first trans-impedance-amplifier configuredto amplify a first differential signal between the first electricalsignal and the second electrical signal, and a secondtrans-impedance-amplifier configured to amplify a second differentialsignal between the first electrical signal and the second electricalsignal, a third trans-impedance-amplifier configured to amplify a thirddifferential signal between the fifth electrical signal and the sixthelectrical signal, and a fourth trans-impedance-amplifier configured toamplify a fourth differential signal between the seventh electricalsignal and the eighth electrical signal.
 33. The optical communicationmethod according to claim 32, wherein a first monitor signal generatoris configured to generate a first monitor signal corresponding to thefirst differential signal from the first trans-impedance-amplifier; asecond monitor signal generator is configured to generate a secondmonitor signal corresponding to the second differential signal from thesecond trans-impedance-amplifier; a third monitor signal generator isconfigured to generate a third monitor signal corresponding to the thirddifferential signal from the third trans-impedance-amplifier; a fourthmonitor signal generator is configured to generate a fourth monitorsignal corresponding to the fourth differential signal from the fourthtrans-impedance-amplifier.
 34. The optical communication methodaccording to claim 9, wherein when the amplitude of the gain controlsignal decreases, a variable optical attenuator increases attenuation ofthe power of the input multiplexed optical signal.
 35. The opticalcommunication method according to claim 9, wherein an analog-digitalconverter is configured to convert the gain control signal into adigital signal.